SQL Server 2016

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Microsoft SQL Server 2016 (SP2-CU12) (KB4536648) - 13.0.5698.0 (X64) 
 Feb 15 2020 01:47:30 
 Copyright (c) Microsoft Corporation
 Express Edition (64-bit) on Windows Server 2019 Standard 10.0 <X64> (Build 17763: ) (Hypervisor)



CREATE TABLE table1 (
   cust_id integer,
   country char(2),
   name varchar(45),
   sales char(2),
   "group" int);
insert into table1 values
   (1,'US','abc','a',100),
   (1,'IN','abc','b',200),
   (1,'DE','abc','c',300),
   (2,'US','xyz','x',100),
   (2,'IN','xyz','y',200),
   (7,'DE','rambo','r',100);
6 rows affected

-- select everyting, and 'US' as first row
SELECT 
   cust_id, 
   country,
   sales,
   CASE WHEN country='US' THEN 0 ELSE 1 END X,
   ROW_NUMBER() OVER (PARTITION BY cust_id 
                      ORDER BY (CASE WHEN country='US' THEN 0 ELSE 1 END)) R
FROM table1
ORDER BY cust_id, CASE WHEN country='US' THEN 0 ELSE 1 END;


cust_id
country
sales
X
R


1
US
a
0
1

1
IN
b
1
2

1
DE
c
1
3

2
US
x
0
1

2
IN
y
1
2

7
DE
r
1
1


-- filter only rows which are first row...
SELECT * 
FROM (
   SELECT 
      cust_id, 
      country,
      sales,
      CASE WHEN country='US' THEN 0 ELSE 1 END X,
      ROW_NUMBER() OVER (PARTITION BY cust_id 
                         ORDER BY (CASE WHEN country='US' THEN 0 ELSE 1 END)) R
   FROM table1
   -- ORDER BY cust_id, CASE WHEN country='US' THEN 0 ELSE 1 END
   ) x
WHERE x.R=1


cust_id
country
sales
X
R


1
US
a
0
1

2
US
x
0
1

7
DE
r
1
1